THE INFLUENCE OF ph ON THE SYNTHESIS OF MIXED FE-MN OXIDE MINERALS

Clay Minerals (1990) 25, 507-518 THE INFLUENCE OF ph ON THE SYNTHESIS OF MIXED FE-MN OXIDE MINERALS M. H. EBINGER* AND D. G. SCHULZE Agronomy Department, Purdue University, West Lafayette, Indiana 47907, USA (Received l August 1989; revised 10 April 1990) ABSTRACT: Mn-substituted iron oxides were synthesized at ph 4, 6, 8, and 10 from Fe-Mn systems with Mn mole fractions (Mn/(Mn + Fe)) of 0, 0.2.0.4, 0.6, 0.8, and 1.0, and kept at 50~ for 40 days. The Mn mole fraction in goethite was <0-07 at ph 4 but increased to -0.47 at ph 6. Goethite and/or hematite formed in Fe and Fe + Mn syntheses at ph 4 and ph 6 at Mn mole fractions <-0.8, and at Mn mole fractions -<0-2 at ph 8 and ph 10. Hausmannite and jacobsite formed at ph 8 and ph 10 at Mn mole fractions ~>0.4. In the pure Mn syntheses, manganite (y-mnooh) formed at ph 4 and ph 6, whereas hausmannite (Mn304) formed at ph 8 and ph 10. As the Mn substitution increased, the unit-cell dimensions of goethite shifted toward those of groutite, and the mean crystallite dimensions of goethite decreased. The substitution of Mn in the crystal structures of iron oxide minerals, particularly goethite, has been the subject of recent research (Karim, 1984; Stiers & Schwertmann, 1985; Cornell & Giovanoli, 1987; Lim-Nufiez & Gilkes, 1987). Karim's study (1984) was conducted at ph 7, and the other studies were conducted at ph > 8. Ebinger & Schulze (1989) showed that Mn-substituted goethite (ol-feooh) and Fe-substituted groutite (ol-mnooh) can also form at acid ph. The experiment described by Ebinger & Schulze (1989) showed that large amounts of Mn can substitute for Fe in the goethite structure, but the factors that control the substitution could not be specifically determined. First, ph was not controlled during the synthesis, and the influence of ph on the formation of Mn-substituted goethite could not be distinguished from the influence of Mn on the formation of the minerals. Second, MnSO4 was the source of the Mn, and 8042- may have altered the rate or the mechanism of formation of the Mn-substituted iron oxides (Dousma et al., 1979; Torrent & Guzman, 1982; Brady et al., 1986). Third, aqueous Mn was lost when the precipitates were washed at the beginning of the synthesis period. This paper presents the results of an experiment designed to determine the influence of ph and Mn addition on the formation of Mn-substituted iron oxide minerals. The experiment was conducted at constant ph of 4, 6, 8, or 10, Fe(NO3)3 and Mn(NO3)2 were the sources of Fe and Mn, and the precipitates were washed only at the end of the synthesis period. The influence of $042- on the formation of Mn-substituted iron oxides will be examined in a later paper. * Present address: Group EES-15, MS J-495, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. 9 1990 The Mineralogical Society

508 M. H. Ebinger and D. G. Schulze MATERIALS AND METHODS Mn-Fe oxides and oxyhydroxides were prepared from stock solutions of 0.5 M Fe(NO3)3 and 0.5 M Mn(NO3)2. The Fe(NO3)3 solution was prepared from commercially available, reagent grade Fe(NO3)3.9H20, and the Mn(NO3)2 was prepared by dissolving Mn metal in 300 ml of a 1 : 2 HNO3 : H20 solution. Stock solutions of Fe and Mn were mixed to give 240 ml volumes with initial Mn fractions of 0, 0.2, 0-4, 0.6, 0.8 and 1.0. Next, 180 ml of 2 M NH4OH were quickly added to each Fe + Mn solution and precipitates formed immediately. The ph of each solution after addition of the base was between 7 and 8. The suspensions were adjusted to ph 4 (samples 1500-1510), ph 6 (samples 2500-2510), ph 8 (samples 3500-3510), or ph 10 (samples 4500-4510) with 1 M HNO3 or 1 M NH4OH within one hour of the initial precipitation, then kept in 1 1 polyethylene bottles at 50~ in a forced convection oven for 40 days. The ph of each suspension was measured and adjusted after one and six hours on the first day, every 12 hours for the next six days, then daily thereafter. The final products were centrifuged, washed three times with deionized water, and dried at 50~ Oxalate soluble Fe (Feo) and Mn (Mno) were determined by extracting duplicate 30 mg samples with 30 ml of ph 3 ammonium oxalate for two hours in the dark (Schwertmann, 1964). Total Fe (Fet) and total Mn (Mnt) in the untreated samples and in the samples treated with oxalate were measured after duplicate 30 mg samples were dissolved in 20-25 ml of concentrated HC1. Fe and Mn in the extracts were determined with a Perkin-Elmer Model 373 atomic absorption spectrophotometer. Self-supporting mounts for X-ray powder diffraction (XRD) were prepared from samples that had been gently ground in an agate mortar. The samples were back-filled into A1 sample holders and gently pressed against unglazed paper. Step-scanned XRD patterns were obtained using Co-Ko~radiation (35 kv and 25 ma) and a Philips PW3100 goniometer equipped with a 1 ~ divergence slit, 0-2 mm receiving slit, and a graphite monochromator. Patterns of all samples were obtained from 3 ~ to 80 ~ 20 at 0.05 ~ increments and counting times of 10 s per increment. Corundum (25% by weight of Buehler Micropolish, Linde C, 1.0 ~m 01-A1203, No. 40-6310-008) was used as an internal standard for position and intensity. Samples from the synthesis at ph 6 were treated with ammonium oxalate for 15 min in the dark to remove poorly crystalline material. The short extraction time was chosen to minimize potential dissolution of Mn-substituted goethite. The unit-cell a, b, and c dimensions of these goethites were determined by Rietveld refinement of the entire pattern (Rietveld, 1969) using the Generalized Structure Analysis System (GSAS) program (Larson & Von Dreele, 1987). Goethite unit-cell dimensions for the untreated samples were calculated by multiple linear regression using line positions measured graphically from expanded plots of the XRD patterns. For samples that contained no hematite, the 110, 130, 111,221 and either the 211, 140, 041, or 121 reflections were used. For samples that contained hematite, the 110, 111, 211, 221, and 020 reflections were used because of overlapping hematite and goethite reflections. The 012 and 104 reflections of corundum were used as internal position standards. Samples 1500-1508, 2500, 3500-3502, and 4500 contained sufficient hematite to determine the a and c dimensions accurately. Unit-cell dimensions were calculated from the 012,014,024, and 124 reflections by multiple linear regression, and the corundum 012 peak was used as an internal position standard.

Synthetic Fe-Mn oxides at constant ph 509 The quantities of goethite and hematite were estimated from the areas under the goethite 110 and the hematite 012 peaks. The peak areas were determined relative to the corundum 012 peak to correct for the different mass attenuation coefficients of the samples. Standard curves were prepared from mixtures of pure, synthetic goethite and hematite. The mean crystallite dimensions (MCDs) of the samples were calculated from the corrected widths at half height of the XRD peaks and the Scherrer equation, as described previously (Ebinger & Schulze, 1989). The goethite standard had an MCD of 17 nm perpendicular to the 110 planes, and the hematite standard had an MCD of 33 nm perpendicular to the 012 plane. Magnetic susceptibility was measured with an analytical balance and a small permanent magnet (Matsusaka & Sherman, 1961). The pan from the analytical balance was replaced with a 50 g counterweight, then re-zeroed. Samples or standards were weighed into a nonmagnetic plastic tube, compressed to a uniform thickness in the plastic tube, and suspended from the bottom of the counterweight. Magnetic susceptibility of a sample was related to the standard as described by Mulay (1963). A sample or a standard was weighed on the analytical balance, then the permanent magnet was placed under the sample and the material was weighed again. The calibration standard was 0.5 g MnSO4.H20. Magnetic susceptibility of the sample, Z,,, was determined from the difference in the two weights by equation 1: xrz~w, wr xs - Aw~ ws (i) where Xr is the magnetic susceptibility of the standard (108.6 10 8 m3/kg for MnSO4.H20); AW~, is the difference in weight of the sample with and without the magnet in place; AWr is the difference in weight of the standard with and without the magnet in place; Wr is the mass of the standard; and W~ is the mass of the sample. The position and orientation of the permanent magnet was the same for each measurement. Oxalate solubility RESULTS Samples with larger Mn fractions had higher oxalate solubilities (Table 1). The Mno/Mnt ratio was smaller than the Feo/Fet ratio for the ph 4 samples, but the Mno/Mnt and Feo/Fet ratios were similar for most of the other samples. The combined oxalate ratio, (Feo + Mno)/(Fet + Mnt), showed several trends. First, products were mostly crystalline in the pure Fe syntheses at ph 8 and ph 10 (samples 3500 and 4500) as indicated by an oxalate ratio of 0.01. However, significant amounts of poorly crystallized material remained at the end of the syntheses at ph 4 and ph 6. The ph dependence shows that the rate of crystallization from ferrihydrite to goethite decreases as the ph decreases from ph 10 to ph 4 (Schwertmann & Murad, 1983). Second, the high oxalate solubilities of the ph 8 and 10 samples are due not only to poorly crystalline ferrihydrite, but also to dissolution of crystalline jacobsite (Fe2MnO4) and hausmannite (Mn203) (see XRD results below). Crystalline jacobsite and hausmannite are soluble in acid ammonium oxalate because Mn is reduced and complexed by aqueous oxalate anion (Chao & Theobald, 1976). Third, the oxalate solubility of the ph 4 and ph 6 samples increased as the Mn mole fraction increased, indicating an increased amount of ferrihydrite with increased Mn.

510 M. H. Ebinger and D. G. Schulze TABLE 1. Mn mole fraction and oxalate solubility data. Mole fraction Mn Sample Initial solution Final After Mn in (Fe + Mn)o/ product oxalate 1 goethite z Feo/Fet 3 Mno/Mnt 3 (Fe + Mn)t Initial ph = 4 1500 0 1502 0-2 1504 0.4 1506 0.6 1508 0.8 1510 1.0 Initial ph = 6 2500 0 2502 0.2 2504 0.4 2506 0.6 2508 0.8 2510 1.0 Initial ph = 8 3500 0 3502 0-2 3504 0.4 3506 0-6 3508 0.8 3510 1.0 Initial ph = 10 4500 0 4502 0-2 4504 0.4 4506 0.6 4508 0.8 4510 1.0 0 4 --0"01 0'28 0 0"28 0"03 -- --0"02 0"29 0"14 0"28 0"04 -- 0"02 0"44 0"25 0"43 0"05 -- 0"05 0"54 0"24 0"53 0"07 -- 0"07 0"53 0"23 0"51 0 0-0-01 0.28 0 0.28 0.18 0.18 0.18 0.36 0.33 0.36 0-22 0.35 0.33 0.61 0.34 0.56 0.23 0-41 0.37 0-74 0.73 0.74 0.36 0.47 0.46 0.75 0.94 0.82 1.00 -- -- -- 1.00 1-07 0 -- 0.00 0.01 0.00 0.01 0.14 -- 0.09 0.20 0.29 0-21 0.27 -- -- 0.90 0.94 0.91 0.40 -- -- 0.82 0.92 0.86 0.62 -- -- 0.74 0.93 0.86 1.00 -- -- -- 0.97 0.97 0 -- -0-02 0.01 0.00 0.01 O. 17 -- 0-08 0.08 0.69 O. 18 0.35 -- -- 0.97 1.06 1.01 0.55 -- -- 0.96 0.98 0.97 0.74 -- -- 1.08 1-01 1-03 1.00 -- -- -- 1.00 1.00 1 After 15 min oxalate extraction. z Mn mole fractions predicted from c dimension regression, equation (2). Negative values indicate zero substitution. 3 Feo, Mno = oxalate soluble Fe or Mn; Fet, Mnt = total Fe or Mn. 4 Dashes indicate no data. X-ray diffraction Mineral phases, ph 4: Both goethite and hematite formed in all of the Fe-containing systems at ph 4 (Table 2). The amount of hematite in the products remained constant as the Mn mole fraction increased, but the amount of goethite decreased. Manganite, 7-MnOOH, was the only mineral that formed in the pure Mn system (sample 1510) ph 6: Both goethite and hematite formed in the pure Fe system (sample 2500) but goethite was the only crystalline phase that formed when both Fe and Mn were present (Table 2). Manganite was again the only mineral that formed in the pure Mn system (sample 2510).

Synthetic Fe-Mn oxides at constant ph 511 TABLE 2. Goethite (Gt), hematite (Hm), jacobsite (Jc), hausmannite (Ha), and manganite (Ma) identified in synthetic products. Gt a Hm 1 X Sample (wt%) (wt%) Jc Ha Ma (x 10 8 m3/kg) ph = 4 1500 25 1502 18 1504 13 1506 10 1508 t4 1510 ph = 6 2500 31 2502 20 2504 19 2506 10 2508 15 2510 ph = 8 3500 38 3502 31 3504 tr 3 3506 3508 3510 ph = 10 4500 55 4502 15 4504 4506 4508 4510 15 _2 203 16 -- -- -- 199 14 -- -- -- 218 13 -- -- -- 293 14 -- -- -- 219 - - - - - - X - - 14 -- -- -- 264.... 440.... 392.... 551.... 280 -- -- -- X 35 37 -- -- -- 34 14 -- -- -- 1183 -- tr X -- 7987 -- tr X - 22764 -- tr X - 10018 -- -- X -- 58 1 4 - - - - -- tr X -- tr X -- tr X - - - - X m m m m m m 29 1673 21660 28281 18562 77 i Percentages refer to the amount of goethite and hematite determined by XRD relative to unsubstituted goethite and hematite standards. Absolute amounts may be in error because the widths at half height of standards and samples were different. : Dashes indicate mineral not present. 3 trace, X indicates the presence of jacobsite, hausmannite, and/or manganite, but quantitative estimates of these minerals were not made. ph 8: Goethite and hematite were the only crystalline phases that formed in the pure Fe system (sample 3500) and in the system that contained 0.2 mole fraction Mn (sample 3502) (Table 2). Hausmannite (Mn304), however, was the predominant phase that formed between Mn mole fraction of 0-4 and 1.0. Crystallization of hausmannite appeared relatively complete in the pure Mn system (sample 3510) as suggested by well-resolved diffraction lines on the XRD patterns (Fig. 1). A considerable amount of poorly crystallized material remained in the systems that contained Mn mole fractions of 0.4 to 0-8 as indicated by broad, poorly-resolved diffraction lines (Fig. 1) and high oxalate solubilities (Table 1). Poorly crystallized material was also observed at an initial Mn mole fraction of 0.2 (sample

512 Synthetic Fe-Mn oxides at constant ph 0 0 ~0 O --ClO O (Dr O ~ 0 D o r c,, I 8 O e-, 0.o O"4" -1 ~.~s -~,2- --v~ ~c'q "r~-- o O.r o e,9 g 'T"

M. H. Ebinger and D. G. Schulze 513 3502) as indicated by a (Fe + Mn)o/(Fe + Mn)t ratio of 0.21 (Table 1). Two weak peaks at 3-00 (34-7 ~ and 2-56 A (40-9~ corresponding to the jacobsite 220 and 311 lines, can be observed in the pattern of sample 3506 (Fig. 1). Jacobsite is highly magnetic (Nagata, 1961) and its identification in sample 3506 by XRD corresponds to the largest magnetic susceptibility measured for the ph 8 and ph 10 samples (Table 2). The large magnetic susceptibilities of samples 3502-3508 relative to the pure Fe and Mn samples (samples 3500 and 3510) suggest that small amounts of jacobsite may have formed in each of the systems containing both Fe and Mn at ph 8 and 10. ph 10: The mineralogy of the ph 10 system is similar to that of the ph 8 system. The presence of Mn had a greater effect on goethite and hematite crystallization at ph 10 than at ph 8. For example, at Mn mole fraction of 0.2, both goethite and hematite formed at ph 8, but at ph 10 goethite was the only crystalline phase that formed (Table 2). Small amounts of jacobsite were present in all Fe- and Mn-containing samples as indicated by magnetic susceptibility data (Table 2), but XRD peaks were poorly resolved. Jacobsite content was again largest at Mn mole fraction of 0.6 (sample 4506) as indicated by a weak 220 peak at 3.00 ~ and a maximum in the magnetic susceptibility. Unit-cell dimensions. The ph 6 samples were used to relate unit-cell dimensions of goethite to Mn substitution because goethite was the only crystalline phase that formed at ph 6 in the Mn + Fe syntheses. Goethite unit-cell a and c dimensions decreased, and the b dimension increased as Mn substitution increased (Fig. 2, Table 3). Each unit-cell dimension deviated from Vegard's Law (Klug & Alexander, 1974). The cell dimensions shifted toward those of groutite, o~-mnooh, and indicate that Mn 3+ substitutes for Fe 3+ in the goethite structure. Each unit-cell dimension was regressed against the Mn substitution to determine statistical relationships between these parameters. The changes in the unit-cell dimensions were linearly related to Mn substitution, and the unit-cell b and c dimensions appear to be the best predictors of Mn substitution (Fig. 2). Mn substitution (XMn) in the goethite of the untreated samples (Table 1) was estimated from the c dimension using the constants from linear regression: XMn = -4.4804 c + 13.538 (2) with r 2 of 0-999, n = 5. The Mn mole fraction of the goethite was similar to the Mn mole fraction in the whole sample at ph 4, but the Mn mole fraction of goethite was greater than in the whole sample at ph 6 (compare Mn mole fraction in "final product" column at ph 4 and ph 6 with "Mn in goethite" column for ph 4 and ph 6, Table 1). Therefore, the oxalate-soluble fraction contains less Mn than the associated goethite at ph 6. Mn substitution in goethite at ph 8 and ph 10 was slightly less than the Mn mole fraction of the whole sample. The hematite a and c dimensions did not change systematically as the Mn mole fraction increased from 0 to 0.8 (Table 3), and suggest that there is no Mn substitution in hematite. Cornell & Giovanoli (1987), however, showed that Mn can substitute for Fe in hematite when hematite forms in the presence of oxalate anion. Mean crystallite dimensions. The MCDlm for goethite from the ph 4 and ph 6 samples increased as the initial Mn mole fractions increased from 0 to 0-6, but it decreased slightly at Mn mole fractions of 0-8 (Table 3). The MCDll 0 of goethite from the ph 8 samples decreased as the Mn fraction increased, and the MCD 110 of goethite from the ph 10 samples remained constant as the Mn increased. The MCDu0 of goethite was larger for the ph 8 and

514 M. H. Ebinger and D. G. Schulze 4.62 o< 4.60 4.58 4.56 4.54 10.50 o<s 10.30 a = 4.6075-0.1255 X u n ~ r 2 = 0.84 i i i i~ b = 9.9536 + 0.9891 Xun 10.10 9.90 3.06 3.02 ~ c : 3.0215-0.2230 Xun f-~ r 2 = 0.999 0<~ -~ ~-~ 2.98 ~ - - _ 2.94 - - - 2.90, T, 0 0.1 0.2 0.3 0.4 0.5 FIG. 2. Unit-cell a, b, and c dimensions vs. measured Mn fraction (XMn). Equations for regression lines as indicated. Vegard law shown by dashed lines calculated from unit-cell dimensions of goethite (card no. 29-713') and groutite (o~-mnooh, card no. 12-733i) from JCPDS (1980). Xun ph 10 samples than for the ph 4 and ph 6 samples, and suggests that goethite crystallites were larger at higher ph. The MCOaal of goethite decreased as the Mn mole fraction increased at all values of ph. However, the MCDul of goethite from the ph 4 samples decreased less than the MCDlu from the ph 6 samples, and indicates that the crystallites were somewhat larger at ph 4 than at ph 6. The ratio MCDu0/MCDH1 is <1 for lath-shaped goethite crystallites, and MCDu0/MCDlu increases as the laths become shorter (Schulze & Schwertmann, 1984). The MCD ratios suggest that unsubstituted goethite crystallites are lath shaped at all phs and become less lath shaped as Mn substitution increases (Table 3), but definitive evaluation of the apparent change in crystallite morphology requires additional evidence such as transmission electron micrographs. The MCD012 of hematite changed little as the Mn fraction increased at ph 4, but the MCD012 of hematite was slightly larger at ph 8 and 10 than at ph 4 and 6. The hematite that formed at ph 6 had the smallest MCD012. Role of Mn in formation of iron oxides DISCUSSION Goethite and hematite formed at all phs in the absence of Mn (Table 2), and maximum hematite formation occurred at ph 8. This is in agreement with Schwertmann & Murad

Synthetic Fe-Mn oxides at constant ph 515 TABLE 3. Unit-cell dimensions and mean crystallite dimensions (MCD) for goethite (Gt) and hematite (Hm). Cell dimensions (A) Gt Hm MCD (nm) 1 Sample a b c a c Gttto Gtm Hmotz Initial ph = 4 1500 4-608 1502 4.587 1504 4-594 1506 4.602 1508 4.596 Initial ph = 6 2500 2502 2504 2506 2508 Initial ph = 6 2500 2502 2504 2506 2508 9.951 3.023 5.035 13.71 26.5 55.2 50.1 9.987 3.025 5.042 13.63 31.5 49.6 43-3 9.978 3.108 5.031 13.74 31.5 45.1 50.1 9.984 3.011 5.031 13.76 37-8 38.1 50-1 10.008 3.007 5.032 13.75 35.0 38.1 47-6 4.606 9.935 3.023 5.034 13.74 23.6 47.2 26.4 4.579 9.884 2.982 2 _ 30.5 20-1 -- 4.551 10.321 2.949 -- -- 31.5 16.8 -- 4.545 10.507 2.939 -- -- 30.5 15.0 -- 4.528 10.463 2.918 -- -- 27.0 12.2 -- (after 15 min oxalate treatment) 3 4-6051 9.9657 3-0205 ***4 *** *** *** _ 4-5864 10.1164 2.9826 -- -- 4.5773 10.2885 2.9443 -- -- *** *** -- 4.5407 10.3611 2-9309 -- -- 4-5512 10.4311 2.9150 -- -- Initial ph = 8 3500 4.605 3502 4.583 Initial ph = 10 4500 4-608 4502 4-583 9.977 3-022 5.035 13.71 41.1 55-1 50-1 10.051 3.001 5-035 13-76 27-0 45-1 68-0 9.979 3.027 5.043 13.69 37.8 58.3 68.0 10.104 3.003 -- -- 37.8 20-7 -- l Mean crystallite dimensions calculated from widths at half height of goethite 110, goethite 111, and hematite 012 reflections, respectively. 2 Mineral not present. 3 Unit-cell dimensions determined from Rietveld refinement using GSAS (Larson & Von Dreele, 1987). 4 Not calculated. (1983) who found maximum hematite formation between ph 7 and ph 8 in a similar synthesis. The presence of Mn, however, had a major impact on the crystalline minerals that formed, and the influence of Mn varied greatly depending on ph. Although exact reaction mechanisms cannot be explained, some insight comes from sorption experiments by Grimme (1968), McKenzie (1980), and Davies (1985, 1986). Grimme (1968) studied sorption of Mn and other heavy metals on synthetic goethite, hematite, and "X-ray amorphous oxide" (ferrihydrite) as a function of ph and found no principle differences between the adsorption properties of the three minerals. Sorption of Mn was minimal at ph < 5 but increased considerably at ph > 5. Only 3% of a 10-5 M Mn solution adsorbed on to goethite at ph 5, but 12% was adsorbed at ph 6 and 50% at ph 7-2 (Grimme, 1968). McKenzie's results (McKenzie, 1980) were similar; there was little

516 M. H. Ebinger and D. G. Schulze adsorption of Mn at ph < 5, about 25% of the maximum sorption occurred at ph 6, and maximum sorption took place at about ph 7.5. Davies (1985, 1986) showed that iron oxide surfaces catalyse the oxidation of Mn +2 to Mn +3 and may also stabilize the highly reactive Mn +3 ion. In the present experiments, Mn and Fe concentrations were considerably higher than in previous studies (e.g., Grimme, 1968). Therefore, comparison of these experiments with the sorption experiments mentioned above may not be entirely valid, but there is good agreement with previous studies and the results reported in the present study. Adsorption of Mn +2, oxidation to Mn +3 on the iron oxide surface, and incorporation into the growing goethite crystatlites is a possible mechanism for Mn-substituted goethite formation suggested by the results of previous studies and by the present study. Manganese appears to have the least influence on the crystallization process at ph 4. Goethite and hematite formed at each Mn mole fraction at ph 4 (Table 2). Goethite formation decreased slightly as Mn addition increased, whereas hematite formation appeared unaffected by the presence of Mn (Table 2). Manganese slowed the transformation of ferrihydrite to more crystalline phases as indicated by an increase in Feo/Fet as the Mn mole fraction increased (Table 1). Mn substituted into the goethite structure in proportion to the amount of Mn added, but the maximum Mn substitution was lower at ph 4 than at higher ph. These results are consistent with the low sorption of Mn by iron oxides at ph < 5 found by Grimme (1968) and McKenzie (1980). Even though Fe and Mn coprecipitated, some of the Mn apparently desorbed from the ferrihydrite at ph 4 and only influenced the rate at which crystalline iron oxide minerals formed. Manganese influences the crystallization process more strongly at ph 6. Manganese slowed the transformation of ferrihydrite to more crystalline phases as indicated by higher Feo/Fet ratios for a given Mn mole fraction at ph 6 than at ph 4 (Table 1). Hematite formation was suppressed completely in the presence of Mn, and goethite formation was decreased (Table 2). The goethite that formed, however, contained up to 0.47 mole fraction Mn, and the amount of Mn substituted in the goethite depends on the initial Mn mole fraction in the solution (Table 1). Greater sorption of Mn by the growing goethite crystals at ph 6 than at ph 4 could account for the greater Mn substitution in goethite at ph 6. The influence of Mn on the crystallization of iron oxides is different at ph 8 and ph 10. Mn-substituted goethite with Mn mole fraction of ~0-08 forms from solutions with initial Mn mole fractions of 0.2 at both ph 8 and ph 10. Hematite formation was suppressed at ph 8 and eliminated at ph 10 (Tables I and 2). Goethite and hematite were eliminated at initial Mn mole fractions >-- 0.4, and hausmannite and jacobsite formed instead (Table 2). Manganese is strongly adsorbed by iron oxide minerals above ph 7-5 (Grimme, 1968; McKenzie, 1980; Davies, 1985, 1986) and may account, in part, for the similarity in the phases formed at ph 8 and ph 10. The actual mechanism is undoubtedly more complex and is beyond the scope of this study. Mn substitution in goethite The syntheses at ph 6 show that goethite with large Mn substitutions (up to 0.47 Mn mole fraction) forms under appropriate conditions. The results of the present study show considerably more Mn substitution than Stiers & Schwertmann (1985) attained at alkaline ph. The results also show that the b and c dimensions are good estimators of Mn substitution in unknown goethite samples in the absence of substitution of other metals

Synthetic Fe-Mn oxides at constant ph 517 (Fig. 2). The c dimension is preferred, however, because it can be calculated from the positions of the 110 and 111 reflections (Schulze, 1984). Aluminum substitution (Schulze, 1984) and Cr substitution (Schwertmann et al., 1989), for example, also cause the c dimension of goethite to decrease linearly. It is impossible, therefore, to distinguish Mn (or Cr) substitution from A1 substitution using the c dimension alone. The b dimension, however, decreases with A1 (and Cr) substitution and increases with Mn substitution, and allows distinction of Mn substitution from A1 or Cr substitution in goethite. The formation of Mn-substituted goethite at ph 4 to ph 10 suggests that Mn-goethite could form under some soil conditions. For example, it could occur in some Fe-Mn concretions from hydromorphic soils. Mn-substituted goethites are probably less abundant in soils than Al-substituted goethites because Mn is less abundant in soils than A1, and because the conditions in which iron oxides form in the presence of Mn are less common than conditions where iron oxides form in the presence of Al. ACKNOWLEDGMENTS We thank the USDA-ARS National Soil Erosion Research Laboratory, West Lafayette, Indiana, for the use of the X-ray diffractometer and P. F. Low (Purdue University), U. Schwertmann (Technische Universit~it Mtinchen), and D. L. Bish and C. Porzucek (Los Alamos National Laboratory) for critical reviews of the manuscript. REFERENCES BRADY K.S., BIGHAM J.M., HAYNES W.F. & LOGAN T.J. (1986) Influence of sulfate on Fe-oxide formation: Comparisons with a stream receiving acid mine drainage. Clays Clay Miner. 34, 266-274. CHAO T.T. & THEOBALD P.K. (1976) The significance of iron and manganese oxides in geochemical exploration. Econ. Geol. 71, 1560-1569. CORNELL R.M. & GIOVANOLI R. (1987) Effect of manganese on the transformation of ferrihydrite into goethite and jacobsite in alkaline media. Clays Clay Miner. 35, 11-20. DAVIES S.H.R. (1985) Mn(ll) oxidation in the presence of metal oxides. PhD thesis, California Inst. Tech., Pasadena, USA. University Microfilms International, #8519538. DAVIES S.H.R. (1986) Mn(II) oxidation in the presence of lepidocrocite: The influence of other ions. Pp. 487-502 in: Geochemical Processes at Mineral Surfaces (J.A. Davis & K.F. Hays, editors). Am. Chem. Soc., Washington, DC. DOUSMA J., DEN OTTELANDER D. & DE BRUYN P.L. (1979) The influence of sulfate ions on the formation of iron(ill) oxides. J. lnorg. Nucl. Chem. 41, 1565-1568. EBINGER M.H. & SCHULZE D.G. (1989) Mn-substituted goethite and Fe-substituted groutite synthesized at acid ph. Clays Clay Miner. 37, 151-156. GRIMME H. (1968) Die Adsorption yon Mn, Co, Cu, und Zn dutch Goethit aus verdiinnten L6sung. Z. Pflanzenernaehr. Bodenkd. 121, 58-65. JCPDS (1980) Mineral Powder Diffraction File. International Centre for Diffraction Data, Swarthmore, Pennsylvania. KARIM Z. (1984) Influence of transition metals on the formation of iron oxides during the oxidation of Fe(II)C12 solution. Clays Clay Miner. 32, 334-336. KEUG H.P. & ALEXANDER L.E. (1974) X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2rid edition. John Wiley & Sons, New York. LARSON A.C. & VON DREELE R.B. (1987) Generalized Structure Analysis System. Los Alamos Nat. Lab. Rep., LAUR-86-748. LIM-Nug~EZ R. & GILKES R.J. (1987) Acid dissolution of synthetic metal containing goethites and hematites. Proc. Int. Clay Conf. Denver, 197-204. MCKENZIE R.M. (1980) The adsorption of lead and other heavy metals on oxides of manganese and iron. Aust. J. Soil Res. 18, 61-73. MATSUSAKA Y. & SHERMAN G.D. (1961) Magnetism of iron oxide in Hawaiian soils. Soil Sci. 91, 239-245. MULAY L.N. (1963) Analytical application of magnetic susceptibility. Pp. 1751-1884 in: Treatise on Analytical Chemistry, vol. 1, part 4 (I.M. Kolthoff & P.J. Eking, editors). Wiley, New York.

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